Drug addiction and relapse to drug taking require associative memory and motivational incentives, processes that critically involve hippocampal formation (HF) output to the mesolimbic reward system. In humans and animals, relapse susceptibility and HF-associated learning behaviors vary by gender, suggesting hormonal as well as sociocultural influences on addiction-linked processes. Separate literatures show that opiate drugs of abuse (e.g., morphine) and ovarian steroids each alter hippocampal function and cognitive performance. Both opiates and ovarian steroids alter HF excitability and long-term potentiation (LTP), the primary model for learning, and neurogenesis that is involved in forms of associative learning. Morphine directly activates mu-opioid receptors (MORs) and affects the availability of delta-opioid receptors (DORs), altering the balance of excitatory and inhibitory circuits. Enkephalins, endogeous HF opioids, activate MORs and DORs and are important in addictive processes. Estrogens and progestins act through both genomic and non-genomic mechanisms to cyclically remodel HF neurons and alter neurotransmission. Understanding interactions of ovarian steroids and opioids/opiates would clarify whether females have unique modulation of drug responses. This renewal application proposes to test the central hypothesis that the hormonal milieu modulates endogenous hippocampal opioid systems and hippocampal responses to exogenous opiates. The proposed studies use electron microscopic immunocytochemistry, autoradiography and in vitro slice electrophysiology in female rats. Aim 1 will determine whether estrogens and progestins: a) affect levels of enkephalins and/or preproenkephalin mRNA in three subregions that critically integrate different afferent information; b) alter the subcellular distribution of MORs and DORs, which are on interneurons that regulate rhythmic output of excitatory projection neurons; and c) interact with opioids to facilitate LTP. Aim 2 will examine whether estrogen and progestin receptors are on: a) enkephalin-containing neurons, suggesting potential direct interactions of these systems; b) hilar mossy cells, major targets of enkephalin terminals and regulators of projection cell output; c) newly generated cells. Aim 2 also will determine if enkephalin-containing neurons, their targets, or newly generated cells contain functional estrogen binding sites. The results will elucidate potential mechanisms and sites where ovarian steroids, by affecting HF opioid systems, may influence hippocampal-dependent learning relevant to drug abuse.